The present invention relates generally to gas turbine engines and more particularly to a multi-spool by-pass turbofan engine capable of serving as a highly efficient auxiliary power unit as well as the main power plant of a jet aircraft.
A modem jet aircraft requires substantial electric power to drive the many accessories required to support ground operation of the aircraft, particularly when the aircraft is parked at the ramp. Normally, power for such accessories is supplied by ground support equipment or by an onboard Auxiliary Power Unit (APU) to avoid the significant fuel consumption, noise, and general disturbance associated with operation of a propulsion engine. While affording maximum flexibility, onboard APU""s have a negative impact on pay load and usable cube of the aircraft. Ground based support equipment is often unavailable.
Jet engines have. heretofore been modified to solve this problem. For example, the twin spool by-pass turbofan engine disclosed in U.S. Pat. No. 5,485,717, and assigned to the assignee of the instant invention, comprises a low pressure spool having a fan at a forward end and a low pressure turbine at the aft end thereof. A coaxial high pressure spool has a high pressure compressor in fluid flow communication behind the fan on the low pressure spool and a high pressure turbine in fluid flow communication behind a combustor and forward of the low pressure turbine on the low pressure spool. Combustion gas is bled into the by-pass duct of the engine from a point upstream of the low pressure turbine thereby to attenuate the speed of the fan.
However, the problem is more difficult when the concept is applied to a three-spool jet engine. Separation of the aerodynamic elements of a by-pass turbofan into three spools permits the speeds of the various spools to adjust themselves in an advantageous manner for off-design operating points. This is particularly true when the overall compression ratio is designed to be relatively high so as to provide the best possible fuel economy and thrust/weight ratio for propulsion purposes. Specifically, the fan is generally oriented forwardly of an annular by-pass duct that surrounds the high, intermediate, and low pressure spools. The fan is driven by the rotation of the low pressure spool. Analysis shows that the speed of the low pressure spool decreases to a greater extent than that of the high pressure spool when the thrust demand is reduced. The lower speed of the low pressure compressor permits it to operate at a lower flow rate without encountering compressor stall. However, optimum performance under all accessory load conditions requires finely calibrated interstage bleed of the high pressure combustion gases.
In the three spool configuration, the intermediate pressure compressor is generally supported on an intermediate spool downstream of the fan and forward of the high pressure compressor. The intermediate spool is driven by an intermediate pressure turbine that is arranged between the high and low pressure turbines. Engine accessories including, for example, a generator, are driven by the high pressure spool. Thus, it is important to maximize the RPM of the high pressure spool while minimizing the RPM of the low pressure spool during ground operation to produce auxiliary power.
The present invention permits a three spool by-pass turbofan propulsion engine to be operated in a programmable finely calibrated benign mode which minimizes fuel consumption and disturbance to personnel on the ramp by developing only the amount of engine power necessary to provide the electrical power, hydraulic power, and compressed air required for the immediate needs of the aircraft. The concept of the present invention involves the provision of an efficient and precisely controllable bleed of the hot gas aft of the high pressure turbine by utilizing mechanical flow diverters disposed downstream of the high pressure turbine but upstream of the intermediate pressure turbine. Poppet valves are used to control the flow diverters and thereby the amount of bleed-off gas discharged into the engine by-pass duct without doing further work. Removal of a substantial portion of the working fluid from flow through the intermediate and low pressure turbines drastically reduces their capacity to power the intermediate compressor and low pressure fan, respectively, thereby reducing both the pressure and temperature of air introduced into the high pressure compressor. Reduced pressure to the high pressure compressor reduces the mass flow therethrough. and to the engine combustor which reduces the required fuel flow while still permitting the high pressure spool to operate at a speed sufficient to drive the engine generator, pumps, etc. Moreover, the reduced high pressure compressor inlet temperature resulting from lower pressures at the inlet thereto reduces the high pressure compressor outlet temperature which, in turn, reduces, or eliminates, the need to cool the air extracted from the high pressure compressor before it can be used as bleed air.
More specifically, in accordance with the present invention, a circumferentially spaced array of poppet valves having flow diverters coupled thereto, is disposed downstream of the high pressure turbine to effect bleed of combustion gas. Use of multiple, highly efficient, individually controlled poppet. valves permits computer control of the amount of combustion gas bleed. Accordingly, bleed can be varied: in discrete, precise increments by opening the valves in a programmed sequence. Controlled bleed permits the low pressure turbine to operate at a speed sufficient to supply only that amount of air to the high pressure section of the engine necessary to generate the power required by on board electrical, hydraulic or pneumatic equipment of the aircraft.
It is to be noted that the broad concept of venting interstage pressure from a point immediately aft of the high pressure turbine into the by-pass duct of the engine when the engine is in the idle condition is disclosed in Williams U.S. Pat. No. 3,363,415, assigned to the assignee of the present invention. In addition, Williams U.S. Pat. No. 5,687,563, also assigned to the assignee of the present invention, discloses the use of digitized computer controlled poppet valves coupled to lever type mechanical flow diverters to vent combustion air away from the intermediate pressure turbine. However, lever-type mechanical flow diverters forced into the fluid flow path by the opening of poppet valves suffer from the disadvantage of causing unnecessary restriction of fluid flow out of the high pressure turbine.
The present invention provides for an array of poppet valve sets arranged circumferentially in the combustion gas duct between the high and intermediate pressure turbines. Each valve set is comprised of three poppet valves. The center valve in each set controls a flow diverter designed to efficiently divert fluid flow-from the high pressure turbine when the valves are in the open position while minimizing fluid flow disturbance when the valves are in the closed position.
An alternate preferred embodiment of the present invention provides for a turbine by-pass bleed system that uses a valve support case connected to a turbine case. The valve support case has a sleeve valve moveable on a top surface of the valve support case. The sleeve valve is openable at engine idle conditions and will open a fluid flow path from the combustion gas duct to the fan by-pass duct of the turbine engine i.e. turbine by-pass bleed mode. When in the closed position the sleeve valve will block any fluid flow from the combustion gas duct to the by-pass duct i.e. when the engine is under full operating conditions.